Defining the T cell receptor has been one of the most persistent problems of modern immunology. Although several studies have shown that T cells express idiotypes similar to those on immunoglobulin (Ig), many other investigations have been unable to document other Ig-associated determinants on T cells. It has been proposed that T cells utilize receptors containing Ig heavy chain variable (Ig VH) domains with different constant regions. The rabbit is a useful species for such studies because it possesses a well-characterized, unique set of Ig VH allotypes that are framework markers for all known rabbit Ig classes. The major obstacle to doing such experiments has been the difficulty in obtaining highly purified functional rabbit T cells. During the past year, this laboratory has developed several continuously proliferating rabbit T cell lines, some of which are functionally active as cytotoxic cells. These cells will be used in experiments with a variety of anti-Ig reagents, including antisera to the Ig VH framework determinants encoded by the a locus allotypes. The results of these studies should confirm or refute the model of the T cell receptor containing an Ig VH domain.